An MRI-based technique for determining three-dimensional deformations in articular cartilage explants

Articular cartilage is critical to the normal function of human diarthrodial joints. The tissue has unique mechanical and tribological properties that allow for locomotion by providing a nearly frictionless and wear-resistant joint surface. Osteoarthritis is the most common rheumatic disease and is associated with various degrees of cartilage degradation and altered joint mechanics. Despite the unique function of normal cartilage and the prevalence of osteoarthritis, previous studies have not quantified three-dimensional mechanical deformations throughout the volume of cartilage using noninvasive experimental methods. The lack of such comprehensive information regarding cartilage deformations seriously limits our understanding of normal cartilage mechanics as well as our ability to completely describe cartilage disorders. A noninvasive Cartilage Deformation by Tag Registration (CDTR) technique was developed to determine three-dimensional heterogeneous deformations in articular cartilage explants. The technique was a combination of specialized MRI methods, a custom cyclic loading apparatus, and image processing software. An optimization analysis found that the absolute strain precision for the technique was a function of MRI parameters and was maximized to 0.41% strain for the range of experimental variables studied. Despite the application of a simple uniaxial cyclic compression to multiple bovine cartilage samples, strain patterns were found to be complex, three-dimensional, and heterogeneous. Strain magnitudes in the thickness direction varied nonlinearly with depth, and in spite of differences in the applied normal stress, strain magnitudes were nearly identical in superficial tissue regions. Strain magnitudes perpendicular to the thickness direction varied linearly with depth and increased with applied normal stress over the thickness of the tissue. Shear strains also exhibited heterogeneous patterns. In general, strain patterns suggested that the cartilage osteochondral explants exhibited a depth-dependent transverse isotropy during uniaxial cyclic loading. These results allow for an understanding of the micromechanical environment likely experienced by individual chondrocytes throughout the tissue volume and may be beneficial for the verification of constitutive formulations of articular cartilage during cyclic unconfined compression. The results additionally represent baseline data that can ultimately be compared to analyses of damaged and surgically repaired cartilage.